Multi-v fm antenna



y 1953 M. w. SCHELDOFF MUL'FI-V FM ANTENNA I5 Sheets-Sheet 1 Filed Sept.24, 1949 [Lit M y'5, 1953 M. w. SCHELDORF 2,637,533

MULTI-V FM AN 'IiENNA Filed Sept. 24, 1949 3 Sheets-Sheet 2 4% wad/Patented May 5, 19 53 MULTI-V FM ANTENNA Marvel W. Scheldorf, PalosHeights, 111., assignor to Andrew Corporation, Chicago, 111., acorporation of Illinois Application September 24, 1949, Serial No.117,628

7 Claims. (01. 25033.53)

The present invention relates to a radio transmitting antenna and moreparticularly, to a transmitting antenna designed for operation in the FMrange of 88 to 108 megacycles.

Many broadcasting companies have obtained permits to transmit in the FMrange in addition to the present AM frequency. It would be highlydesirable to locate the FM transmitter at the AM transmitter site and touse the AM antenna tower for the FM antenna.

In making provision for a simple, lightweight and inexpensiv FMtransmitting antenna, it would be desirable to provide an arrangementwhereby the antenna elements might be readily attached to the sides orcorners of existing AM antenna towers or to a relatively short mast tobe mountedat the top of an existing AM antenna tower. Such requirementsnecessitate an asymmetric construction which however, should provide ina horizontal plane a substantially circular radiation pattern.

In accordance with the present invention, a relatively simple,lightweight and inexpensive transmitting antenna for operation in the FMrange is obtained by providing a plurality of bays, each of which has ashape corresponding generally to a truncated V. In an antenna of thistype however, it is desirable to provide a driving impedance having asubstantial value of the order of 50 ohms so as to minimize the matchingproblem in joining the antenna radiators to conventional transmissionlines.

In order to realize further economies in the construction of an FMantenna, it is possible to provide radiating elements which areadjustable. Generally, however, such adjustable arrangements result inchanges in both the horizontal radiation or field intensity pattern andvariations in antenna impedance. Some of the pattern variations are notsufficiently severe but what they can be tolerated. However, impedancevariations and changes require additional adjustable elements such as amultiplicity of matching elements or tuneable stubs. In accordance withthe present invention however, it has been found possible to tune anantenna for resonance for any desired frequency of transmission withoutany substantial change in the antenna impedance value. A single matchingdevice has been found sufficient to transform the impedance at thejunction point of the antenna structure and the transmission line inspite of an inherent characteristic whereby the driving impedance of theantenna varies over the frequency range. By means of a matchingstructure, it has been possible to obtain an impedance at the junctionpoint which averages 51.5 ohms over the FM frequency range.

Since it is customary in FM transmitters to employ a plurality of bays,the present invention contemplates the use of as little as two bays,each having a conservative power rating of 5 kw.

It is therefore, an object of the present invention to provide a simple,lightweight and inexpensive transmitting antenna designed for operationin the FM range of 88 to 108 megacycles.

It is a still further object of the present invention to provide an FMtransmitting antenna having a substantially constant field intensitypattern throughout the FM range without utilizing a multiplicity oftuning and matching devices.

It is a still further object to provide a multiple bay antenna having arelatively simple antenna feed system.

Still another object of the present invention is to provide a simpleassymmetric FM antenna having a substantially circular horizontal fieldintensity pattern.

A still Iurtner object of the present invention is to provide an FMantenna arrangement to permm insulation resistance checks withoutdisconnectlng the antenna radiating elements.

Still another object of the present invention is to pIOVlQe a simple FMantenna which may be attached to exlstlng antenna tower structures.

A still further OlJJeCt of the present invention is to provide an FMantenna design suitable for mass production whereoy a single design maybe ad usted or pre-tunea at the Iactory for any frequency of operatlon.

Still another object of the present invention is to provide an lmprovedBM antenna structure whlcn may employ a plurality of bays and which hasa standing wave ratio ranging lrom 1.2 to 1.4 in the range of as to luzimeg'acycles.

Other and further objects or the present inventlon subsequently W111become apparent from reference to the lollowlng description taken inCOnJHIlCDiOH with the drawings. wherein:

Figure 1 is a perspective view of a mounted Flvl antenna embodying theprinciples or" the present invention;

bigure z 1s a eetailed view partially in cross section of the upperantenna bay;

b Figure 3 1s a bottom view or the upper antenna Figure 4 is a detailedview partially in cross sec non of the lower antenna bay;

Figure 5 is a graphical representation of the horizontal held intensitypattern;

Figure 6 i a schematic showing or the electrical circuit of the antenna;and

Figure 7 is a schematic showing of a modified electrical circuit for theantenna.

Most existing antenna structures which are to serve for the transmissionof both FM and AM radiations are adapted to support an additional membersuch as an I or H beam ll shown in Figure 1. The support mast Hpreferably is a steel H beam so oriented that the well on one side formsa protective enclosure for a transmission line !2 whereas the oppositeside or well of the H beam serves as enclosure for a power conductor l3shown in Figure 3. The H beam H carries at its top a beacon light HZ ofany suitable type which meets the requirements of the FederalCommunications Commission regulations. In order to service the light andthe antenna radiators, the H beam is provided with a plurality of stepsit; which preferably are placed in those portions or the walls ofthe'well containing the power conductor l3.

For the purpose of illustrating the present invention there has beenshown in Figure l, a transmitting antenna embodying an upper bay IE anda lower The upper bay i5 preferably is carried by a support plate iiiwhich is secured in position by a plurality of straps l9 extendingacross the dances of the H beam H. The strap construction, subsequentlyto be described, permits vertical movement of the upper bay iii to allowfor the difierence in expansion of the copper transmission line i3 andthe steel H beam H. Such an arrangement also permits the mast ii to bendwithout unduly straining or stressing the transmission line [2.

The lower bay ii is also mounted on a supporting plate 2i which may beretained in position by a strap or'if desired the plate 2! may berigidly connected to the H beam 1 i.

Each bay consists of two radiating elements arranged in the form of atruncated V. Each element is preferably formed of a telescopic structurewhich is most conveniently formed of a plurality of tubes. The largertube has a length which is selected for the highest frequency operationof an antenna of this type within the I nected to the outer conductor ofthe coaxial transmission line 52. The outer extremity of this largertube of one of the radiators is also connected by aconductive element tothe outer conductor of the coaxial transmission line l2. The otherlarger tube of the radiating elements is connected by a conductive strapto the inner conductor of the transmission line [2.

Certain details of the antenna structure will become more apparent byreference to the detailed figures. In Figure 2, there is shown the upperbay structure which is supported by a mounting plate It. The mountingplate 18 carries the stud 23 which is retained in position by aplurality of bolt or cap screws 2 The stud 23 supports the outer tube orprimary radiating element 25. This tube '25 has a length which isdetermined by the highest frequency radiation in an antenna of this typewithin the FM frequency range. The outer extremity of the tube 25 isconnected to a conductive strip or power lead 26 which terminates at abinding post 21 carried by an insulator 28. The tube 25 carries anextension tube 29 arranged to telescope relative to the tube 25 so thatthe overall effective length of the radiating element provides a tuned 4radiator at the transmitting frequency. The outer extremity of the tube29 may be sealed by a cap or other closure member 3 l.

The insulator 28 shown partially in cross section carried by a plate 32retained in position by suitable cap screws 33 which pass into threadedopenings in the mounting plate 8. The mounting plate i8 is provided witha suitable opening for receiving a junction box 3d which may be securedin position by any suitable manner including silver soldering. In orderto provide a weather-proof and gas tight seal an O ring is interposedbetween the mounting member 32 and the junction box 34.

From the cross sectional View of the insulator 28 it will be seen thatthe insulator has a hollow portion which carries a central conductorstem 36 arranged to receive a banana plug The banana plug 37 is abifurcated cylindrical portion of a conductor which is secured to aconductor 3d. The conductor 39 is insulated by a plurality of beadedinsulators l! and held in spaced coaxial relation to the inner conductor42 or the coaxial line 12. The inner conductor 42 of the transmissionline section 53 is mounted on spaced insulator beads 3 carried withinthe outer conductor d5. The conductive member 39 extends within theinner conductor 32 of the transmission line section it? a sufficientdistance to form a capacitor of the proper value. The outer conductor 15terminates in a mounting flange Eli which is coupled by suitable bolts4'! to a cooperating mounting flange carried by the other section of thetransmission line H2. The series capacitor interposed between the innerconductor c2 and the transmission line and the primary radiating element25 serves to produce several advantages. Since one element of each bayis insulated by a capacitor it is possible to make resistance checkswithout disconnecting the radiating elements. The series capacitor alsoprovides positive reactance elements in the direct driving impedance sothat the antenna element is operated at a frequency several megacyclesabove resonance. Operation at a frequency above resonance stabilizes thehorizontal field intensity pattern.

By reference to Figure 3 it will be seen that the primary radiatingelement :38 i provided with a telescopic portion 419 which has an endcap 5|. The outer extremity of the larger conductor 48 is connected by aconductive strip 52 to the mounting plate l8 by means of a cap screw 53.From this it becomes apparent that each of the fixed extremities of theradiating elements is connected to the mounting element is which is inelectrical conductivity with the outer conductor 55 of the coaxialtransmission line section 43.

The details of the construction of the lower bay are apparent fromFigure 4. The mounting plate 2| carries a stud 54% held in position by asuitable cap screw 55. The stud 54 carries the larger conductor or tube56 which supports a telescopic portion 51 having its end sealed by a cap58. The outer extremity of the primary conductor 56 of the radiatingelement is connected by a conductive strip 59 to the binding post 61carried by the insulator 62. The insulator 62 is supported by a plate 63secured in position by suitable cap screw 84 which engages threadedholes in the mounting plate 2!. The mounting plate 2| adjacent theinsulator 62 is provided with a suitable opening for receiving a portionof the junction box 65 which is retained in position by suitablefastening means as by silver between the junction box 65 and themounting or support plate 63 of the insulator 62 to provide a gas tightseal.

lhe insulator 62 which has a hollow portion is provided with a centralconductor 61 having a hollow extremity for receiving the banana plug end68 of a conductive conductor 69. The conductive conductor Bil extendsthrough an insulated bushing H which is fitted into an aperture H in theinner conductor 13 of the coaxial transmission line section. Theconductive conductor $9 is connected to a conductor rod M which issupported in coaxial relation to the inner conductor 13 by means of aplurality of insulating beads 15. The conductive rod 14 together withthe inner conductor 13 of the coaxial transmission line forms acapacitor which thereby insulates the radiating element from thetransmission line so that resistance measurement may be made withoutdisconnecting the antenna elements.

The inner conductor 13 is mounted in coaxial relation with respect tothe outer conductor 16 by means of a plurality of insulating beads orsupports Tl. Because of the junction box 65 the outer conductor has aportion cut away to receive this junction box. The junction box at isprovided with suitable grooves which are filled with silver solder 18 soas to provide a gas tight connection between the junction box and theseveral portions of the outer conductor Hi. The outer conductor 16 ateither end is provided with mounting flanges F9 for connection tocooperating sections of the coaxial transmission line. The upper andlower bays are spaced one Wave length apart and hence sufficient coaxialconductor is employed to provide this spacing with the frequency atwhich the antenna is to operate.

In order to match the impedance of the an tenna to the impedance of thecoaxial conductor which interconnects the lower bay with thetransmitter, the inner conductor 13 is provided with a sleeve 8|. Thesleeve 8! is a quarter wave length long. The impedance of the presentantenna construction is high as compared to other constructions sincethis construction involves the folded dipole principle. The use of thisprinciple minimizes unbalanced currents. Furthermore, the higherimpedance makes it possible to feed only one half of the system withenergy. Hence, as previously described, one of the radiating elementshas one end at an intermediate point thereon connected to the outerconductor of the transmission line. It will also be noted that inantennas of this type, where the effective operating length is about onequarter wave length, the highest voltage appears at the outer extremityor the open end. Therefore, it is desirable to keep these open ends asfar apart as possible for all frequency values. Due to the truncated vconstruction, these desirable requirements are met and there has beenprovided an excellent safety factor. Thus, any tendency toward flashover is minimized.

In the development of the present antenna, it was found that the presentconstruction avoids some of the disadvantages occuring in assymmetricalantenna structures. It has been found that the present antenna providesa power gain of 1.6 to 1.8 and a favorable standing wave ratio of 1.2 to1.4 throughout the frequency range of 88 to 108 megacycles.

Referring to Figure 5 of the drawing, there is shown a typicalhorizontal field intensity pattern by the curve A in relation to thephysical orientation of the radiating elements. Measured radiationpatterns show that the greatest deviation from circularity at anyfrequency is 1.5 decibels.

may be made without disconnecting the radia- 'Ihese resistancemeasurements tion elements. test the insulation resistance of the feedline.

An antenna constructed in accordance with the present invention is tunedat the factory by adjusting the telescopic members radiating elements ofeach bay. 'Ihese members are then soldered in place to hold the tuning.If, at any time, the frequency of operation of the transmitter is to bechanged, the antenna may be readily tuned by unsoldering the telescopicmembers, ad usting them accordingly and resoldering them in position.

To permit movement of the base Id of Figures 1, 2 and 3 relative to theH beam ll, the base it is held in position by two pairs of straps i9.Each strap terminates in an apertured portion bent at right angles tothe body thereof so that suitable bolts or cap screws can be insertedthrough the apertures. at one end of each strap engages the base it. Atthe other end of each strap the bolt or cat screw engages an anchor studplate 22 which is welded at right angles to the flange of the H beam II.For convenience in manufacture and installation, it is preferable tohave each bay supported in this manner, and this construction is ofdefinite advantage where more than two bays are installed.

While a prelerred arrangement for single or multiple bay antennas hasbeen described and shown in Figures '1 to 6, another arrangement may beemployed. This other arrangement is shown schematically in Figure 7 andmay be of advantage where an economical single bay antenna is desired.In such case the construction shown in Figures 2 and 3 would be modifiedby eliminating the conductors 26 and 52, and by mounting the radiatingelement #3, 19 on an insulator so that the end of the tube 48 adjacentthe insulator would be connected to the binding post 21.

While for the purpose of illustrating and d..- scribing the presentinvention a particular embodiment has been illustrated in the drawings,it is to be understood that the invention is not to be limited thereby,since such variations in the construction and in the instrumentalitiesemployed are contemplated as may be commensurate with the spirit andscope of the invention as defined in the accompanying claims.

What I desire to protect by United States Letters Patent is claimed asfollows:

1. An asymmetric antenna system having a generally circular horizontalradiation pattern comprising a single transmission line connected to aplurality of bays, each bay comprising a pair of generally horizontalconductors arranged at an angle to each other to form a truncated V,

in the The bolt or cap screw ,7. the fixed end of. eachof. said".conductors. connected to one of said conductors. of said. transmissionline, means connecting the other conductor of said transmission line toa. point intermediate the ends. of onezoi the other horizontalconductors, and a. capacitor interposed between said transmission lineand said secondmentioned conductor of each. bay.

2. An asymmetric antenna system. hating av generally circular horizontalradiation pattern. comprising a single coeaxial transmission=v line.connected to. a plurality of. radiating bays, each bay comprisinga pairof. telescopic membersarranged at an angle to each other in a generallyhorizontal plane so as to form a; truncated. v, the fixed endof each ofsaid conductors being connected to an outer conductor of said coaxialtransmission line, means interconnecting the in-v ner conductor of saidtransmission line with a point intermediate the ends of one of saidhorizontal conductors. of each bay, a capacitor interposed between saidinner conductor of said transmission line, and said latter mentionedconductor of each bay, andan impedance match-- ing section connectedimmediately beneath the lower main bay.

3. An antenna consisting of a plurality of bays connected to a. coaxialtransmission line, each bay comprising a pair of generally horizontallinearly extending radiating elements mounted onaba'se to formatruncated V, one-of said elements being connected to theouter conductoroi ati'ansmission line and the other element being connected through acapacitor tothe inner 6011- ductor of the transmission line, said baysbeing spaced a wavelength apart, an H beam for supporting said bases,and means connecting said bases to said H beam for limited verticalmovement relative thereto.

4. An antenna comprising a coaxial transmission line. a pair ofgenerally horizontal radiat ing conductors mounted" upon a base, saidconductors being arranged at an acute angle to each other to form atruncated V, thefixed end of one of said conductors being c-onnected tothe outer conductor of said transmission line, and means connecting theinner conductor of trans mission line to saidother horizontal conductor.

5. An antenna comprising" a coaxial transmis-- sion line, a pairofgenerally horizontal radiating conductors mounted upon a base, each ofsaid. conductors being formed of two" telescopic members foradjustment-in accordancewith the frequency to be transmitted, saidconductors being arranged at an angle to each other to form a-truncatedV, the fixed end of one ofsaid-condoctors being connected to the outerconductor of said transmission line, and means connecting the: innerconductor of said transmission line to a point, intermediate the ends ofsaid other horizontal conductor.

6. An antenna comprising a coaxial transmission line, a pair ofgenerally horizontal radiating conductors mountedon a base, each of saidconductors being formed of a fixed member and a movable member arrangedtelescopically for adjustment in accordance with the frequency to betransmitted, said conductors being arranged at an angle to each other toform a truncated V, the fixed end of one of said conductors beingconnected to the outer conductor of said transmission. line and theinner conductor of said transmission line being connected to a pointintermediate the ends of said other horizontal'conductor adjacenttheouter end of. the-fixed mem-- ber thereof.

7. An asymmetric antenna. having a. generally circular horizontal;radiation pattern compris ing a coaxial transmission line connected toa. plurality of bays, each bay comprising a pair ofv linearlyextendinghorizontal radiating conductors arranged at an angle to eachother to form a truncated v, each of said conductors being formed of twotelescopic members for adjustment in accordance with the frequency to betransmitted, the fixed end of. each of said conductors being connectedto the outer conductor of said transmission line, and capacitor meansconnecting the inner conductor of said transmission line to said otherhorizontal conductor intermediate the ends thereof adjacent the outerend of the fixed supporting section of said telescopic member.

MARVEL W. SCI-IELDORF.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,911,234 Meyer May 30, 1933 2,111,743 Blumlein Mar. 22, 19382,131,108 Lindenblad Sept. 27, 1938 2,175,363 Roberts Oct. 10, 19392,258,406 Carter Oct. 7, 1941 2,259,623 Fener Oct. 21, 1941 2,267,951Roosenstein Dec. 30, 19.41 2,272,608. Hoffman Feb. 10, 1942 2,324,462Leeds July 13, 1943 2,420,967 Moore May 20, 1947' 2,467,962 Caraway Apr.19, 1949: 2,480,154 Masters Aug. 30, 1949 2,514,020 Wehner July 4,19502,516,500 Alford July 25,1950] OTHER REFERENCES Radio-Electronics,August 1949, page 68'.

